An attempt has been disclosed to incorporate a reversibly thermochromic material having color-memorization properties in a toner (see, e.g., Patent Document 1).
With copies obtained by applying the above toner, they have the function of selectively giving a view of either of two states before and after discoloration but, in a practical use of the copies, they do not necessarily exhibit the satisfactory function of keeping the color tone in an ordinary temperature region and, at some environmental temperature in winter or summer, they in some cases fail to exhibit expected function due to discoloration of the toner itself or discoloration of the copies.